1. Field of the Invention
The present invention relates to a sensor element, and in particular, to a sensor element with which particular pressures in the vacuum range can be measured.
2. Discussion of Background Information
The measurement of vacuum pressures plays a growing role in technology, particularly in high technology, where process control under vacuum has become very widespread. Examples where this type of process control is applicable include: material coating and material quenching and tempering, semiconductor technology, and the food industry.
For measuring low pressures, particularly in the vacuum range, different measuring principles and structures are used. A group of vacuum sensors utilizes the pressure dependence of the damping properties of gases for determining pressure. Sensors of this type are called friction vacuum gauges.
A first moving body located in the vacuum to be measured is typical for all friction vacuum gauges. A second moving or fixed body is usually attached thereto in the immediate vicinity so that a narrow gap is formed between the two. Due to various damping effects, the movement of the first body is damped, wherein the damping is pressure dependent. The detection of the damping can then take place in different ways.
For example, the Spinning Rotor Gauge (SRG) is commercially available from MKS Instruments, Inc. With this friction vacuum gauge the time is measured in which a ball held magnetically in a horizontal plane and rotating about a perpendicular axis is decelerated from an initial speed to a fixed final speed by the gas molecules surrounding it and colliding with it. This time is a measure of the pressure to be measured. The disadvantage of this friction vacuum gauge is that it is a complex arrangement susceptible to faults, has relatively large dimensions and has maximum measurable pressure values of 1 mbar.
WO 02/04911 teaches a friction vacuum gauge in the form of a tuning fork. Compared to other embodiments, such as, e.g., a seismic mass suspended in an oscillating manner, as shown in DE 430 09 893, the arrangement of the oscillator results in low energy transmission or energy discharge to the support body of the oscillator and thus a pressure range expanded to lower pressures of 10−3 to 100 mbar. Despite this measure, the measurable pressure range is not satisfactory.
According to EP 0735354, expansion of the measuring range can be achieved by the arrangement of two separate sensors or one sensor that utilizes two orthogonal oscillation modes for, respectively, one lower pressure range from 1.33×10−6 to 1.33×10−2 mbar, and one higher pressure range from 1.33×10−3 to 1.33×103 mbar. With both arrangements the signals must be evaluated by either two sensors or two modes, which require a complex evaluation electronics system.